Oxygen vacancies as active sites for water dissociation on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy experiments and density functional theory calculations, we determine unambiguously the active surface site responsible for the dissociation of water molecules adsorbed on rutile TiO2(110). Oxygen vacancies in the surface layer are shown to dissociate H2O through the transfer of one proton to a nearby oxygen atom, forming two hydroxyl groups for every vacancy. The amount of water dissociation is limited by the density of oxygen vacancies present on the clean surface exclusively. The dissociation process sets in as soon as molecular water is able to diffuse to the active site.

Bonding of gold nanoclusters to oxygen vacancies on rutile TiO2(110)

Through an interplay between scanning tunneling microscopy (STM) and density functional theory (DFT) calculations, we show that bridging oxygen vacancies are the active nucleation sites for Au clusters on the rutile TiO2(110) surface. We find that a direct correlation exists between a decrease in density of vacancies and the amount of Au deposited. From the DFT calculations we find that the oxygen vacancy is indeed the strongest Au binding site. We show both experimentally and theoretically that a single oxygen vacancy can bind 3 Au atoms on average. In view of the presented results, a new growth model for the TiO2(110) system involving vacancy-cluster complex diffusion is presented.

Effects of Nb doping on the TiO2 anatase-to-rutile phase transition

We study the influence of Nb doping on the TiO2 anatase-to-rutile phase transition, using combined transmission electron microscopy, Raman spectroscopy, x-ray diffraction and selected area electron diffraction analysis. This approach enabled anatase-to-rutile phase transition hindering to be clearly observed for low Nb-doped TiO2 samples. Moreover, there was clear grain growth inhibition in the samples containing Nb. The use of high resolution transmission electron microscopy with our samples provides an innovative perspective compared with previous research on this issue. Our analysis shows that niobium is segregated from the anatase structure before and during the phase transformation, leading to the formation of NbO nanoclusters on the surface of the TiO2 rutile nanoparticles.

Nanosized Nb-TiO2 gas sensors derived from alkoxides hydrolization

Nanocrystalline TiO2 modified with Nb has been produced through the sol-gel technique. Nanopowders have been obtained by means of the hydrolysis of pure alkoxides with deionized water and peptization of the resulting hydrolysate with diluted acid nitric at 100 C. The addition of Nb stabilizes the anatase phase to higher temperatures. XRD spectra of the undoped and the Nb-doped samples show that the undoped sample has been almost totally converted to rutile at 600 C, meanwhile the doped samples present still a low percentage of rutile phase. Nanocrystalline powders stabilized at 600 C with grain sizes of about 17 nm have successfully been synthesized by the addition of Nb with a concentration of 2% at., which appears to be an adequate additive concentration to improve the gas sensor performances, such as it is suggested by the catalytic conversion efficiency experiments performed from FTIR measurements. FTIR absorbance spectra show that catalytic conversion of CO occurs at lower temperatures when niobium is introduced. The electrical response of the films to different concentrations of CO and ethanol has been monitored in dry and wet environments in order to test the influence of humidity in the sensor response. The addition of Nb decreases the working temperature and increases the stability of the layers. Also, large enhancement of the response time is obtained even with lower working temperatures. Moreover, humidity effects on the gas sensor response toward CO and ethanol are less important in Nb-doped samples than in the undoped ones.

Nanosized Nb-TiO2 gas sensors derived from alkoxides hydrolization

Nanocrystalline TiO2 modified with Nb has been produced through the sol-gel technique. Nanopowders have been obtained by means of the hydrolysis of pure alkoxides with deionized water and peptization of the resulting hydrolysate with diluted acid nitric at 100 C. The addition of Nb stabilizes the anatase phase to higher temperatures. XRD spectra of the undoped and the Nb-doped samples show that the undoped sample has been almost totally converted to rutile at 600 C, meanwhile the doped samples present still a low percentage of rutile phase. Nanocrystalline powders stabilized at 600 C with grain sizes of about 17 nm have successfully been synthesized by the addition of Nb with a concentration of 2% at., which appears to be an adequate additive concentration to improve the gas sensor performances, such as it is suggested by the catalytic conversion efficiency experiments performed from FTIR measurements. FTIR absorbance spectra show that catalytic conversion of CO occurs at lower temperatures when niobium is introduced. The electrical response of the films to different concentrations of CO and ethanol has been monitored in dry and wet environments in order to test the influence of humidity in the sensor response. The addition of Nb decreases the working temperature and increases the stability of the layers. Also, large enhancement of the response time is obtained even with lower working temperatures. Moreover, humidity effects on the gas sensor response toward CO and ethanol are less important in Nb-doped samples than in the undoped ones.

Síntesi de nanopartícules de TiO2 fàcilment processables i el seu ús per a fabricar a baixa temperatura cel·les solars sensibilitzades

La gran demanda energètica i la problemàtica dels combustibles fòssils i d‟altres recursos per a obtenir energia posen de manifest la necessitat de desenvolupar tecnologies netes, sostenibles i econòmicament viables de generació d‟energia. En aquest àmbit, les cel·les solars sensibilitzades amb colorant (Dye Sensitized Solar Cells, DSSC), que transformen l‟energia solar a energia elèctrica, són una solució factible. A més, el desenvolupament de mètodes per a construir aquestes cel·les a baixa temperatura permetria fabricar-les sobre substrats plàstics, fet que els donaria un valor afegit i permetria una producció en continu, ràpida i amb baix cost tant, econòmic, com d‟impacte ambiental. Aquest treball presenta el desenvolupament d‟un mètode de producció a baixa temperatura (140 ºC) de DSSC, amb eficiència de 5,9 % sobre substrats FTO/vidre. Aquest valor és superior a la majoria de les eficiències reportades a la bibliografia de DSSC construïdes a baixa temperatura. Les capes mesoporoses que formen els elèctrodes de les DCCS es dipositen per doctor blade a partir d‟una pasta composta per nanopartícules de TiO2, de 4-8 nm, recobertes d‟àcid 3,6,9-trioxadecanoic, per nanopartícules de Degussa P25, de 20-25 nm, i com a dissolvents només s‟utilitza aigua i etanol. L‟aplicació d‟un tractament a 140 ºC permet eliminar la matèria orgànica de la superfície de les nanopartícules de 4-8 nm i unir-les a les de Degussa P25. Aquest fet permet obtenir capes mesoporoses sinteritzades de 6 μm de gruix. A més, l‟aplicació d‟un post-tractament, en el qual s‟utilitza l‟àcid hexafluoro titànic (IV), produeix un lleuger increment en l‟eficiència. Endemés, l‟obtenció de capes primes de TiO2 sobre substrats plàstics és un tema d‟actualitat a causa de la falta de mètodes de deposició a baixa temperatura. En aquest context, s‟ha sintetitzat, mitjançant processos respectuosos amb el medi ambient nanopartícules de TiO2 cristal·lí modificades superficialment amb lauril gal·lat i àcid 3,6,9-trioxodecanoic. Les nanopartícules poden ser dispersades en dissolvents orgànics i aigua respectivament, fet que permet obtenir suspensions estables i de fàcil manipulació. Aquestes poden ser utilitzades com a precursores per a obtenir capes primes a baixa temperatura de TiO2 cristal·lí. En concret, les capes primes formades per nanopartícules de TiO2 modificades amb àcid 3,6,9-trioxodecanoic s‟han utilitzat com a blocking layer en les DSSC construïdes a baixa temperatura.

Uso de TiO2 dopado con nitrógeno para la generación de hidrógeno bajo irradiación con luz visible

Se desarrollaron catalizadores de TiO2 dopados con nitrógeno para conseguir actividad fotocatalítica bajo irradiación visible. En este trabajo se reporta la síntesis de TiO2-N, usando urea y nitrato de amonio (NH4NO3) como precursores de nitrógeno, tanto a partir de un TiO2 generado in situ (método sol-gel) como mediante la modificación de un TiO2 comercial. Así mismo, se varió la concentración de urea para encontrar el valor óptimo de nitrógeno, lo cual se comprobó mediante la oxidación fotocatalítica de ácido oxálico bajo irradiación con luz visible. Los materiales sintetizados se caracterizaron por medio de análisis elemental, y por reflectancia difusa UV-visible, encontrándose nitrógeno en todas las muestras, y un valor del ancho de banda prohibida en el rango 2-2,8 eV. Lamentablemente, se detectó una pérdida de nitrógeno cuando los fotocatalizadores eran reutilizados, lo cual causó una disminución de su actividad después de cada reacción, ya sea en presencia de oxígeno, o en ausencia de éste mientras se generaba hidrógeno. Entre los dopantes investigados el NH4NO3 mostro una mejor eficiencia en la producción de hidrógeno. Además, los resultados experimentales revelaron claramente que la deposición de platino en la superficie de los catalizadores TiO2-N desempeña un papel fundamental en el aumento de la generación de hidrógeno. Sin embargo, esta mejora dependía claramente del método de preparación de las muestras, obteniéndose mejores resultados con el TiO2-p25.

Síntesis, caracterización y ensayos cinéticos del nanocomposite RuO2/TiO2/MWCNT/Pt en la producción fotocatalítica de combustibles solares bajo iluminación UV y VIS

Se prepararon partículas de nanocomposites basados en TiO2 y nanotubos de carbono multicapa platinizados para la obtención de combustibles solares. Se evaluó la actividad fotocatalítica del material en la producción de hidrógeno, en procesos de degradación de ácido fórmico, y en la obtención de hidrocarburos a partir de la reducción de CO2 en agua. Los nanocomposites fueron sintetizados por medio de la técnica sol-gel. Se estudió el efecto de la proporción y el diámetro de los nanotubos de carbono en la actividad del material bajo iluminación ultravioleta y visible. Se estudió el efecto de la adición de RuO2 (0,5% wt.) en la actividad bajo iluminación visible. Los materiales fueron caracterizados por ATR, XRD, BET, HRTEM y SEM. Se obtuvieron sólidos macroporosos, con contenido de fase anatasa superior al 99% y tamaño cristalino comprendido entre 15 y 21 nm. Los resultados cinéticos mostraron una producción óptima de hidrógeno para el composite TiO2/(5wt.%)MWCNT/Pt(60-80 nm), con eficiencia cuántica y eficiencia energética de 1,27% y 0,27%, respectivamente. En el caso de radiación visible, la producción de hidrógeno fue nula para los composites TiO2/MWCNT/Pt, mientras que para el sistema RuO2/TiO2/MWCNT/Pt se observó que la adicción de MWCNT inhibía la actividad fotocatalítica del composite RuO2/TiO2 en la región del visible. Por otra parte, en los ensayos de reducción de CO2 no se detectó ningún producto de reacción.

Structural and functional characterization of (110)-oriented epitaxial La2/3Ca1/3MnO3 electrodes and SrTiO3 tunnel barriers

La2/3Ca1/3MnO3 (LCMO) films have been deposited on (110)-oriented SrTiO3 (STO) substrates. X-ray diffraction and high-resolution electron microscopy reveal that the (110) LCMO films are epitaxial and anisotropically in-plane strained, with higher relaxation along the [1¿10] direction than along the [001] direction; x-ray absorption spectroscopy data signaled the existence of a single intermediate Mn3+/4+ 3d-state at the film surface. Their magnetic properties are compared to those of (001) LCMO films grown simultaneously on (001) STO substrates It is found that (110) LCMO films present a higher Curie temperature (TC) and a weaker decay of magnetization when approaching TC than their (001) LCMO counterparts. These improved films have been subsequently covered by nanometric STO layers. Conducting atomic-force experiments have shown that STO layers, as thin as 0.8 nm, grown on top of the (110) LCMO electrode, display good insulating properties. We will show that the electric conductance across (110) STO layers, exponentially depending on the barrier thickness, is tunnel-like. The barrier height in STO (110) is found to be similar to that of STO (001). These results show that the (110) LCMO electrodes can be better electrodes than (001) LCMO for magnetic tunnel junctions, and that (110) STO are suitable insulating barriers.

Spin-glass behaviour in mixed metal oxides with a rutile-type structure

We report here on the magnetic properties of compounds of composition Fe1−xCrxSbO4 and Fe1−xGaxSbO4. The introduction of paramagnetic Cr3+ and diamagnetic Ga3+ into the rutile‐related iron antimonate lattice does not destroy the antisite atomic ordering which exists in iron antimonate of composition FeSbO4. The initial slope of the Curie temperature dependence on x is similar in both series, indicating that Fe3+‐Cr3+ interactions are very small. The magnetic susceptibility measurements recorded from the compounds of composition Fe1−xCrxSbO4, x<0.4, and Fe0.9Ga0.1SbO4 show them to behave as spin glasses at low temperatures. The inhibition of compounds of the type Fe1−xCrxSbO4, x>0.4, and Fe1−xGaxSbO4, x>0.1 to undergo a spin‐glass transition above 4.2 K is associated with a dilution effect.

Ab initio study of MF2 (M=Mn, Fe, Co, Ni) rutile-type compounds using the periodic unrestricted Hartree-Fock approach

The ab initio periodic unrestricted Hartree-Fock method has been applied in the investigation of the ground-state structural, electronic, and magnetic properties of the rutile-type compounds MF2 (M=Mn, Fe, Co, and Ni). All electron Gaussian basis sets have been used. The systems turn out to be large band-gap antiferromagnetic insulators; the optimized geometrical parameters are in good agreement with experiment. The calculated most stable electronic state shows an antiferromagnetic order in agreement with that resulting from neutron scattering experiments. The magnetic coupling constants between nearest-neighbor magnetic ions along the [001], [111], and [100] (or [010]) directions have been calculated using several supercells. The resulting ab initio magnetic coupling constants are reasonably satisfactory when compared with available experimental data. The importance of the Jahn-Teller effect in FeF2 and CoF2 is also discussed.

Comparació entre dos mètodes d'estiraments ; estàtic i dinàmic, realitzats en l'escalfament de curses i entrenaments de 60, 100 i 110 metres tanques

Aquest estudi es basa en l'idea de que l'escurçament de la cadena musuclar posterior és una causa important del mal d'esquena en nens. És per aquest motiu que es planteja un tractament per a prevenir l'escurçament de la cadena muscular posterior i el mal d'esquena en forma de sessions de Stretching Global Actiu, mètode basat en la Reeducació Postural Global del mateix Ph.E. Souchard, que es posarà en pràctica en 3 escoles diferents de la comarca d'Osona. L'estudi té una durada de 3 cursos escolars (3r, 4t i 5è de primària) en el qual els nens seleccionats per al grup experimental (n=30) realitzaran classes de Stretching Global Actiu dues vegades a la setmana durant 3 mesos en cada curs escolar. Tres escoles més formaran el grup control (n=30) i s'espera que els nens que formen part del grup experimental millorin la flexibilitat de la cadena musuclar posterior (això es podrà observar a través dels resultats del test "sit and reach") i que no presentin mal d'esquena en el transcurs de 3 anys (durada de l'estudi).

Cationic and charge segregation in La2/3Ca1/3MnO3 thin films grown on (001) and (110) SrTiO3.

Electron energy-loss spectroscopy is used to map composition and electronic states in epitaxial La2/3Ca1/3MnO3 films grown on SrTiO3 001 and 110 substrates. It is found that in partially relaxed 110 films cationic composition and valence state of Mn3+/4+ ions are preserved across the film thickness. In contrast, in fully strained 001 films, the Ca/La ratio gradually changes across the film, being La rich at film/substrate interface and La depleted at free surface; Mn valence state changes accordingly. These observations suggest that a strongly orientation-dependent adaptative composition mechanism dominates stress accommodation in manganite films and provides microscopic understanding of their dissimilar magnetic properties.